1. Field of the Invention
The present invention relates to focus adjustment and focus evaluation methods and apparatuses in which the previously mentioned methods are applied to lens systems, particularly wide angle lenses which have large magnification ratios such as the lens system used in an endoscope.
The present invention further relates to an image sharpness evaluation method, a sharpness evaluation method for focus adjustment, a focus adjustment method and focus evaluation apparatus, and a focus adjustment apparatus.
The present invention further relates to a screen apparatus which is applied to the evaluation and adjustment methods and apparatus.
2. Description of Related Art
Generally, conventional art relates to the examination and evaluation of the focusing conditions or image sharpness of an image forming optical system, such as:
(1) using a screen with slits projected so that the spreading out of the slits can be measured as an indication of an out-of-focus image, PA0 (2) a Modulation Transfer Function (MTF) derived from slit images using Fourier transform analysis, and PA0 (3) a maximum resolution frequency derived from a repetitive patterned screen applied as evaluation standard parameters. PA0 (A) Interference between a repeated pattern of a screen and the repeated pattern of a fiber optic bundle; PA0 (B) Interference between the light receiving element of the image transmitted by a fiber optics and the repeated pattern of the fiber optic bundle; and PA0 (C) Interference between the image signals of the light receiving element and the image, in the case where the image has a repeated pattern.
Using conventional method (1) and (2) it is not possible to distinguish between focusing errors and errors due to the large magnification being used by the optical system (e.g. endoscope).
The above mentioned MTF computation also has difficulties of operational time consumption in the Fourier conversion process and the cost of examination tends to increase dramatically if a high speed computation means is required.
Conventional method (3) requires that the image size of the slit screen is always the same, while the use of different magnifications and different object positions lead to a need to have different slit screens for each permutation of object distance and magnification to be used by the apparatus. Choosing the correct screen for the correct conditions is difficult and obviously complicates the method.
Conventional optical evaluation systems provide other methods such as: MTF values being measured by generating an interference pattern light wave fronts; measuring the diffusion of a point light source or a line light source image; measuring the resolving power; projecting a test screen image in the reversed direction along the optical axes of the apparatus and examining it from the object position. Yet another method for the focal point examination is to introduce a parallel light beam from a collimator onto the focal plane, the light beam corresponds to an object at infinity focus.
In a conventional apparatus, e.g. an endoscope, which has fiber optic bundles and a light receiving element (e.g. CCD), it is found that an overlapping image may be formed due to both the light receiving element and the optical fiber bundle being made from a number of elements. This causes moire interference fringes when measuring the resolving power with a test screen which makes such test results unreliable. There are many factors that contribute to the generation of moire interference fringes, among these the following factors are major ones:
In addition, the image transmitted through an optical fiber bundle is not formed by light dots within which the transmitting light is at a constant brightness intensity level. The brightness intensity of the light dots of the fiber optical core follow a Gaussian distribution (brightness intensity drops in proportion to the distance from the center of the light dot), and in the surrounding cladding material of the fiber optic, some light leaks through to other fibers causing noise. The data obtained with the above mentioned dot images include the repeated interference patterns of the cores and cladding and the inherent repeated interference noise.
Yet another known optical evaluation and adjustment method for pan-focus optical systems, such as an endoscope, is to use a collimator and a screen. The screen is placed at the nearest and farthest distances expected, for the given application, and the system is adjusted to give the same amount of defocus at the two extremes. The collimator produces parallel rays incident upon the objective lens.
In still another conventional test method, a screen is moved to various object distances and the focusing accuracy and other optical performances are tested by observing the images formed on a light receiving element by the object of evaluation at points corresponding to different object distances.
The previously mentioned method which utilizes parallel rays, cannot evaluate the focusing positions or optical performances of objects placed at close focusing distances.
The moving screen method mentioned above has further drawbacks, such as a long test time required, and unstable test results, as there are screen positioning errors in distance and inclination at each test position.
A resolving power test screen, which has three or more slits, has yet other difficulties. When positioned at the various object distances, the image magnification also varies. To maintain the same image size on a surface of the light receiving means, various sized screens must be provided to allow a choice corresponding to the image magnification. Conventionally, the images of several screens cannot be formed on the surface of the light receiving means simultaneously and therefore the screen must be replaced at each test sequence.
The above-described conventional methods are obviously not suitable where precise focusing and optical image quality is required, such as in the case of an endoscope where an object is placed from a few millimeters to around ten centimeters, or a compact camera where the distance to the object is from around ten centimeters to infinity, or as in a security camera where the distance to the image is from a few meters to tens of meters.
Further, using the above-described focusing adjustment methods, a good compromise in the image quality in terms of the object distance cannot be made. For example, in one case, the image quality is more than satisfactory at a far object distance while the image quality is not acceptable at a close object distance. In another case, a reversed phenomenon can be observed wherein the image quality is better than required at a close distance while the image quality at a far distance is not acceptable.
The amount of defocusing of an out of focus image changes with a change in the image magnification. An out of focus image having a low magnification looks less significant than an out of focus image having a high magnification at a far distance.
An endoscope, applied in the medical field in a large number of applications, for example, observations of the esophagus, stomach, intestines or other digestive organs, requires precise focus from a few centimeters to over ten centimeters, while the endoscope used in, for example, the observation of lungs and bladders, requires a good focus for only a few millimeters but is not required at all to have good focus at more than a few centimeters.
The focusing and optical performance is subjectively judged by the medical doctors who use such apparatus, and therefore, the conventional far and close focus balance, in which the amount of out-of-focus at the image plane appears similar regardless of the object distance, does not necessarily satisfy the apparatus users.
Optical focus adjustments and image evaluation methods, which are well known, use a method in which the focus is adjusted against a collimator which provides parallel rays and moving screens.
Unfortunately, this method using parallel rays can not evaluate the focus position or the image quality for close object distances. The moving screen method takes a long time for evaluation and spatial and angular errors in positioning the moving screen cause a low consistency in repeated precise testing.
In the case of a resolving power test screen, which uses screens with multiple lines and patterns, the image magnification changes as the object distances change. To obtain the same size of image at the light receiving element, a number of different sized screens must be provided and an appropriate one should be selected for each test. Moreover, in the conventional systems, screens are chosen for standard test distances which are not necessarily relevant to the actual users preferences, and because of such testing methods the focusing point and image quality is not at the user's preferred position.
Yet another difficulty of the conventional evaluation methods, is the applicability of standard evaluation test methods to apparatus which has optical performances well outside the standard testing range.